The present inventon relates to a muting circuit for an AM stereophonic receiver. More particularly, the invention relates to a muting circuit for an AM stereophonic receiver which employs an AM stereophonic technique wherein the carrier wave is phase modulated by a sub signal or the difference signal of the two stereo channels.
Several techniques have been proposed for stereobroadcasting in the AM radio frequencies. One of these is the AM/PM or AM/FM technique wherein a carrier signal is phase modulated by a sub signal (L-R) then amplitude-modulated by the main signal (L+R) for transmission. However, this technique is disadvantageous in that an interferring signal can phase-modulate the carrier signal with the percentage of modulation easily exceeding 100% and sometimes reaching as high as severl hundred percent. For example, in one AM/PM system, 100% modulation of the carrier signal by the sub signal corresponds to a phase angle of 1 radian (about 57 degrees). An interferring signal can easily shift the phase of the carrier signal by more than 57 degrees. Therefore, if an FM detector having "S-curve" characteristics is used to detect the sub carrier from such modulated signal, a rectangular pulse signal of a high level is produced which causes an audio output noise that is very unpleasant.